The present invention relates to a composite barrier membrane coated on a substrate or device and the membrane is formed by phase inversion technique utilizing a water-insoluble wax and a water-insoluble polymer dissolved in a solvent. The barrier membrane coated upon a substrate is used to control transmembrane transport of a liquid, such as water, and a gas, such as oxygen and nitrogen.
Barrier membranes are used to control transfer, by diffusion or sorption, of substances in a gas or liquid phase from a region of higher concentration to one of lower concentration, where concentration means mass of substance contained in a unit volume. The dialysis process is a familiar example in which movement of a solute is restricted by a semipermeable membrane such that low molecular weight material passing the membrane is separated from a mixture of low and high molecular weight material where the membrane is impermeable to the latter. The rate of transfer of a substance across a membrane with time depends upon differential concentration across the barrier and upon permeability properties of the membrane itself Thus, variability in membrane intrinsic properties controls transmembrane transfer rates. For a substance Q and a particular membrane the coefficient of diffusion (xcex94) is defined by the equation:             ⅆ      Q              ⅆ      t        =            -      Δ        ⁢          xe2x80x83        ⁢          (                        ⅆ          c                          ⅆ          x                    )        ⁢          ⅆ      y        ⁢          ⅆ      x      
where dQ is an amount of Q passing through a membrane in the direction of x in time dt, membrane area is dydx and dc/dx is the rate of increase of concentration of Q in the direction of x. For transfer of water in the gas phase or water vapor across a membrane dc/dx is proportional to partial pressure of water in the gas phase or to concentration. It is an objective of this invention to control membrane composition and permeability characteristics expressed in xcex94 such that transfer of water and gasses across the membrane is predictably controlled.
Hagenmaier and Shaw (Hagenmaler, R D, and Shaw, P E, xe2x80x9cMoisture Permeability of Edible Films Made with Fatty Acid and (Hydroxypropyl) Methylcellulose,xe2x80x9d J. Agric. Food Chem., 1990, 38, 1799-1803) prepared a film or membrane by evaporation cast from (hydroxypropyl) methyl cellulose in 95% ethanol solution containing 0 to 65% C12 to C18 fatty acids (1). The barrier membranes were designed to be edible and to serve as a device for controlling moisture transfer in food packaging. The article reported a study of transmembrane transfer of water vapor or permeability and found this parameter to be strongly dependent upon both the concentration of fatty acid dissolved in the solution used to fabricate membranes and the carbon chain length of the chosen fatty acid. Stearic acid most efficiently reduced water transfer when its concentration was increased to about 45% by weight in parent methylcellulose-fatty acid ethanol solution. Membranes composed of hydrophilic polymers prepared by this method possessed a major disadvantage in that water vapor permeability was greatly increased at values of relative humidity above 94%.
Moisture content greatly influences shelf life of many foods and thus food science research laboratories have taken a leading role in development of methods for forming polymer membranes which incorporate hydrophobic materials to confer resistance to water transfer where this property is useful to preserve flavor or texture. In U.S. Pat. No. 4,915,971 to Fennema, Kamper and Kester, an edible film prepared from hydrophilic polymer and fatty acid laminated with an additional layer of beeswax were used, for example, to separate a watery pizza sauce component from surrounding bread component to prevent sogginess during storage. The patent describes the preparation of an edible film by first dissolving hydrophilic polymers such as starch, albumin or cellulose methyl and hydroxypropyl ethers in water solution, adding ethanol and a blend of stearic and palmitic acids at 65xc2x0 C. and plating the resulting solution onto glass for drying at 100xc2x0 C. An additional lipid layer was added in the following step by pouring on a thin coating of molten beeswax at 180xc2x0 C. or by adding the beeswax from an ethanol solution. After careful removal from the glass support, this membrane was useful as a water transfer barrier for hydrophilic pizza sauce during storage; the membrane was absorbed into the bread layer upon cooking or preparation at temperatures exceeding 70xc2x0 C. Alternatively, the membrane could be formed directly on a food surface when fatty acids were omitted from the composition of the base polymer layer (3).
Greener, et al. (Greener, I K and Fennema, O., xe2x80x9cEvaluation of Edible, Bilayer Films for Use as Moisture Barriers for Food,xe2x80x9d J. Food Science, 1989, 54, 1400-1406) showed that addition of a beeswax lamina in a second step was a major improvement over prior polymer-fatty acid membranes. Water vapor transfer measured at 97% relative humidity was reduced four-fold where the lipid layer of film was positioned towards the high relative humidity side of a test apparatus. Investigation of resistance to oxygen transfer of these laminated bilayer films by Greener, et al. showed that beeswax membranes provided gas barrier comparable to commercial packaging materials such as mylar and cellophane (5).
Nisperos-Carriedo, et al. (Nisperos-Carriedo, M O et al., xe2x80x9cChanges in Volatile Flavor Components of Pineapple Orange Juice as Influenced by the Application of Lipid and Composite Films,xe2x80x9d J. Agric. Good Chem. 1990, 38, 1382-1387) studied retardation of spoilage in oranges by applying surface wax-based coatings. Membranes formed by brushing various mixtures of sucrose esters of fatty acids, carboxymethyl cellulose sodium salt, diglycerides, beeswax and propylene glycol esters of fatty acids showed increased in components, such as acetaldehyde, methyl butyrate, and ethyl butyrate, which are major contributors to fruity top-notes of orange flavor. Changes in these components probably resulted from altered metabolic pathways as affected by permeability of the coatings to carbon dioxide and oxygen although the authors made no effort to control transfer of these gasses.
Broadly, one aspect of the present invention pertains to a synthetic integral composite membrane coated upon a substrate, a device, or a support layer by deposition from a one-phase composition, specially constituted casting, or membrane fabrication solution containing a water-insoluble polymer, a water-insoluble wax and a solvent with addition of optional additives.
One objective of this invention is to fabricate an integral composite membrane of wax-polymer, a membrane not found in nature and which has barrier properties and can limit or retard transport of a liquid, a gas, or water vapor.
A further objective of this invention is to prepare or fabricate or cast the composite membrane on a substrate by a one-step phase inversion deposition procedure which would be convenient for industrial manufacture.
Another objective of this invention is to systematically alter wax and polymer composition of the membrane casting or fabrication composition from which a membrane is deposited so as to generate films which have predictable transmembrane transport properties for liquid, water vapor or gas.
Another objective of the present invention is to coat a starch-based biodegradable polymer substrate or support material with an appropriate integral composite membrane to control water transfer such that the substrate takes up minimal water from a food or other product on storage but remains degradable under more extreme conditions.